Coil for relays and inductive apparatus



3, 1943- c. M. MORRIS 2,326,053

COIL FOR RELAYS AND INDUCTIVE APPARATUS Filed Aug. 28, 1941 5 I E 3 .9 E .0 .7

Q .6 g Q 5 g u S .4 l

TURNS lNl ENTOR By C. M. MORRIS A T TORNE V that the magnetic Patented Aug. 3, 1943 COIL FOR RELAYS AND INDUCTIVE APPARATUS Charles M. Morris, East Orange, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.', a corporation of New York Application August 28, 1941, Serial No. 408,513

9 Claims.

This invention relates to coils for relays and other types of inductive apparatus and has for its object the securing of a high degree of longitudinal inductive balance.

Relays with two windings are used in large numbers in telephone systems with their two windings connected to the tip and ring conductors of the talking circuit and it is essential for quiet speech transmission that there shall be as small an inductive unbalance between the two windings as possible. Unbalances in relays now in general use have been found to be as high as 25 microamperes per volt.

Three different types of double wound relays and retardation coils have been used heretofore, the first of which is the tandem or front and rear winding type consisting of two coaxially adjacent windings each of which occupies the entire depth and approximately one-half the length of the winding space. Assuming that the two windings have an equal number of turns and similar dimensions, as is usually the case, it is obvious that the inductive balance is perfect provided structure is symmetrical lengthwise with respect to the two windings. However, practical considerations usually prescribe an unsymmetrical structure in which case the inductance of the two windings is inherently unbalanced. With any such actual structure, the effective symmetry and therefore the inductive unbalance are functions of the applied saturation.

The secondtype heretofore employed comprises two concentric windings each extending over the full length of the winding space. The chief advantage of this winding arrangement is the simplicity of manufacture, while the chief disadvantage is the rather poor balance under varied conditions of permeability of the magnetic structure If a perfect magnetic structure, that is, one whose permeability approaches infinity, be assumed, it is then obvious that the inductive balance approaches perfection if the windings have an equal number of turns since all of the flux links all of the turns of each winding. When, however, the permeability of this type of structure is unity, that is, when an air core is used, the inductive balance is poor since the inductance of the outer winding is greater than that of the inner winding because the former has a larger radius. With the magnetic core material now used in relays of this type and with the various conditions of direct current saturation encountered, the inductive balance will, of course, lie between these two extremes. In

general, however, the permeability for such relays will be no greater than ten even at the lowest voice frequencies encountered and the balance will be unsatisfactory for most types of transmission circuits- The third type is known as sandwich wound and comprises three concentrically disposed windings, each extending over the full length or" the winding space. The inner and outer windings have an equal number of turns and are connected in series to in effect constitute a single primary winding and the middle or secondary winding has a number of turns equal to or substantially equal to the sum of the turns of the other two windings. It is obvious that a coil wound in this manner is more symmetrical radially than the coil with two concentric windings and is more symmetrical lengthwise than the coil with tandem windings. In general therefore with any practical magnetic structure, a higher degree of inductive balance may be obtained with the sandwich type coil than with the tandem or with the two concentric winding coils. If the coil is provided with primary and secondary windings having an equal number of turns and rose oi the primary winding are equally divided between the inner and outer windings, the primary and secondary windings are well-balanced provided that the magnetic structure has high permeability, but not so well balanced if the permeability is low. In general this winding arrangement of the coil is poorly balanced for high values or" direct current saturation. If the inner and outer windings constituting the primary winding have equal turns and the middle or secondary winding has about one per cent less turns than the sum of the turns of the other two windings, a better balance is secured since the induetances of the primary and secondary windings are more nearly equal at average values of direct current saturation, but an improvement in balance with this type of coil is still possible and desirable.

In accordance with the present invention, a higher degree of inductive balance is secured by providing an improved type of sandwich wound coil in which the inner and outer windings which constitute a single primary winding have, in general, an unequal number of turns, and the middle or secondary winding has a number of turns equal to the sum of the turns of the other two windings. Also the sum of the di ect current resistances of the inner and outer windings equals the resistance of the middle winding.

' mary and secondary windings along the length of ances, N1,- N2 and N3 be assumed be the 1 To satisfy both conditions 1 and 2 hereiriec referred to, it is necessary for 2' v amen 'Referringnow to the drawing, V such as have been determined by the United Fig. 1 shows a coil, partly in cross section; hav- States Bureau of Standards. If as expected the ing its three windings proportioned in accordsubstitution of these values in Equation 4 does ance with the invention, and I not yield the original assumed value of.

Fig. 2 showscurves graphically illustrating the 5 l N1, longitudinal unbalance of variously wound coils. V T73 Coils wound in accordance with the present invention meet the following four conditions Which are necessary in order to insure optimum balance, namely: (1) 7 perfect core balance; 19 (2) air core balance; (3) symmetry of the priadditi'onaltrials must be. made until the com puted and assumed values substantially agree.

Since the primary and secondary windings are concentric and wound to the same length, they are obviously symmetrical along the length of the magneticstructure and, therefore; meet the the ma netic structu e and, (4) direct current V g u 1 third aforementioned conditlon.

resistance balance. Fig. 1 illustrates acoil l wound in accordance with the ion mount ed on a core 2 and having t -ce widings E, 4 Q and 5 wound concentrically and positioned on the core 2 between the fiber spoolheads and l;

The numeral 3 designates the inner windi g, 3'

the middle or secondary Winding and 5 the outer 20 winding. windings '3' and 5 are ccnnectabie in series to constitute fliefpriinary winding of the coil. l f l If L1, L2 andl'La be assumed to be the inductand' secondary windings may be approximately balanced to meet the fourth condition byselecting the proper wire gauges for each of the three windings. If this is not surficiently close the direct current resistance may be balanced by adding a few ncneinductive turns of resistance wire; I I

i applied experimentally toe, batteryfsup 1y relay coil for usein a transmission circuit of stepi by step telephone system where a shi degree "of longitudinal balance required. From 'a circuit cperatienstandpoint its is that the relay windings have a total number of turns of turns of the windings'li, i1 and e,' respect and 1013 the coefficient or" cotipling'be'tween the inner'winding 3 and the outer winding '5, the

is 5'; "esse'd e roiiowgigig gg balance e m between 10,009 and 11,609 and a total due-ct cur- V v r 7 I r rent resistance Rdc between 360 and L +L 2lc a /L L =L I r (1) these objectives to be secured with an approximately full wound coil spool. V i ,1 7

A single approximation made in accordance with the method hereinbefore describe-died;-

cated .a value for now let apart-n L=a2N2 L3=a3IN3 wn To 0.1, a2 and 213 "are determined by the dimensions Substituting these values in Equat;

following equation is obtained: fi z;

'1', the V I N 2 a N2=rh+2v3 V 3) 7 7 f E Substituting quation 3 in Eeua-ticn '2, rear mac/mm Opm'mum Value? has deter fahgmgiand sgivmg for i I a mined experimentally by measuring the unbal- 7 7 l .ance of afew relays the coils of wh-ich were A rwoueclwithdifi rent va f l N;, I V I 7 N1 the following equation issec ured: a 7 s 7 WP,

each coil with a total .of 11,000 turns and with No. 34. enamel covered wire for the inner wind- 1 ing, No. 33 enamel covered wire-for the middle Q 1 I V winding and No. 32 enamel covered wire for the Ni V outer- Winding. The specifications for the five to have the value given by "Equation 4 when the @0115 are Expressed in follewing table:

factors 1e13, a1; a2 and us have their air core g V s values. Since these iactors are constant onlyn v V Primsrlylwmding for a particular set of dimensions, it is necesl- I I middle sary to solve for 7 I i V p v N r I Inner Outer ,To-1anrl-7 I I I 3 i A N2 inc 7 ,v I. ,4 N1 113 Na'ltd N NIB by a process of successive approximations; i-l'l the design of relays'er retard coils other re- 3 5 560 V 194 quireme'nts usually the. number oiv turn-s; 7% '31 190' 123 51500 "197 51500 197 The usual procedure :is: to assume a value of p g gggi 1 y .2 l V I 11 5 2 ll1 7 7O 80 2900 c 0500 we 5, 00 I to A3 The curves of -Fig.- 2 have been plotted from which with the total mutter er *iis N1, values of longitu'dina l imbalance roneaeher the N2 and N3. Further assumption of the wire "fivefexperirnentai relay coils ever the voice fregauges fixes 'th'e dime'nsl-ions so that the four quencyifer 0 and 180 nulliamperesuirect curiactors' may be computed by means of formulae- 75 r m-,tggmmt m' fihgs. -ive t 11 be The direct current resistance of the primary The improved winding arrangement been 7 noted that the minimum unbalance occurs at approximately N1=24O0 turns. As might be expected the minimum for miilampere saturation is less well defined than for 100 milliamperes satration since the former conditions more nearly approximate the perfect core in which the balance is independent of ATI to form a primary winding and the middle wind ing serving as a secondary winding and having a number of turns equal to the sum of the turns of the inner and outer windings.

6. In an electromagnetic device, a magnetic structure and a coil having three concentric windings 01 coextensive length, the inner and outer of which have a ratio of number of turns derived by the formula It has thus been demonstrated that the minimum unbalance may be secured by propcrtioning the turns of the inner and outer windings so that the outer winding has the greater number of turns.

What is claimed is:

1. A coil for relays and similar devices having 2. A coil for relays and similar devices having three concentric windings of coextensive length, the inner and outer of which have an unequal number of turns and are connectable in series to form a primary winding and the middle winding serving as a secondary winding, and having a number of turns equal to the sum of the turns of the inner and outer windings.

in which (11, a2 and a3 are determined by the dimensions of the windings and by the magnetic structure and R213 is the coefficient of coupling between the inner and outer windings, said inner and outer windings, being connectable in series to form a primary winding and the middle windin servin as secondar winding hevin three concentric windings, the inner and outer o g a y a number of turns equal to the sum of the turns of the inner and outer windings.

7. In an electromagnetic device, a magnetic structure and a coil having three concentric windings, the inner and outer of which have a ratio of turns derived by the formula 1 2" mi s i 'J lil l fi is m 1 3 w s 1 2 i a in which 011, 0:2 and at; are determined by the dimensions of the windings and by the magnetic structure and 7613 is the coefficient of coupling between the inner and outer windings, said inner and outer windings being connectaole in series to form a primary winding and the middle winding serving as a secondar winding and having a number of turns and direct current resistance 3. A coil for relays and similar devices having 40 equal to the sum of the turns the of the three concentric windings, the inner and outer of which have an unequal number of turns and are connectable in series to form a primary winding and the middle winding serving as a secondresistances, respectively, of the inner and outer windings.

8. In an electromagnetic device, a magnetic structure and a coil having three concentric ary winding, and having a number of turns and i5 windings of coextensive length, the inner and direct current resistance equal to the sum of the turns and the sum of the resistances, respectively, of the inner and outer windings.

4. A coil for relays and similar devices having three concentric windings of coextensive length,

outer of which have a ratio of number of turns derived by the formula in which a1, a2 and a3 are determined by the dimensions of the windings and by the magnetic structure and kn is the coeflicient of coupling the inner and outer of which have an unequal between the inner and outerwindings. said inner number of turns and are connectable in series to form a primary winding and the middle winding serving as a secondary winding, and having a number of turns and direct current resistance equal to the sum of the turns and the sum of the resistances, respectively, of the inner and outer windings.

5. In an electromagnetic device, a magnetic structure and a coil having three concentric windings, the inner and outer of which have a ratio of number of turns derived by the formula and outer windings being connectable in series to form a primary winding and the middle winding serving as a secondary winding and having a number of turns and direct current resistance equal to the sum of the turns and the sum of in which a1, a2 and 013 are determined by the diing connectable in series to form a primary wind mensions of the windings and by the magnetic structure and R13 is the coefficient of coupling between the inner and outer windings, said inner and outer windings being connectable in series ing and the middle winding serving as a second ary winding and having a number of turns equal to the sum of the turns of the inner and outer windings.

CHARLES M. MORRIS. 

